In a cd there was some old photos, But while again loading some other photos in nero by mistake we have given multisession. The problem is all the old photos have been overwritten and we are not able to view that photos. Very rare photos which cannot be taken again. What to do. Please help me with this problem. The cd is not a rewritable one.

Had a quick look at the web site suggested by HairyWill and it seemed as if the free trial version was limited to recovering files of up to 64K. You have to pay for the full version to recover bigger files.

Another Windows solution, which does not have this limitation, is Isobuster. I am sure the free version is not meant to be used this way but it can be done with a little trickery. Not for the complete novice.

Have a look at my review of Isobuster and what I did when I lost my photos by looking here:
http://www.icpug.org.uk/national/features/061001fe.htm

Are normal burnt CD's simply 'overwritten? Or, does the next session simply go alongside the first and somehow obliterate the first string character, like a deleted document on the hard drive?

This may or not be relevent and is just a suggestion but, try looking here:
http://www.cgsecurity.org/wiki/CDRW#Read_of_previous_cdrom_session

Quote:

Read of previous cdrom session

With multi-session cdrom, it is possible to delete files of previous session. Because the files are not really deleted, it is possible to recover them. To read files from the first session, run under Linux

Hi
All may not be lost quite yet. It sounds to me like the last session you burned on that CD didn’t link the other tracks together quite right. Nero is good at that. I don’t know how to retrieve your files with Linux but in Windows all you have to do is put the CD in your CD/DVD drive, then open up “My Computer” , right click on the drive > click on properties > click on the “Volumes tab” and double click on the track(s) prior to the last track, that is read by default. Doing this will close the last track and open the track (volume) you choose so it will be the one being read. Open Explorer and you should see the files that were on the disk before the last “bum” burn. Copy all the files onto your HD and start over with a new CD, but don’t forget to drag over the files from the last track too if you want to save them also.
Hope this works for Ya.

BTW: A Multisession CDRW and CDR are written in the same way (by track) but a CDRW can rewrite over tracks but normally not until it is deleted out of the fat on the CD.